One known method for manufacturing a carboxylic ester from an aldehyde involves first manufacturing a carboxylic acid by oxidizing an aldehyde, and then reacting this carboxylic acid with an alcohol. The problem with this method, though, is that the reaction consists of two steps, so the equipment cost is high, and furthermore the yield is inadequate in the step of oxidizing the aldehyde. In particular, in the manufacture of methacrylic acid by the oxidation of methacrolein, the selectivity is only about 80% at best, so the space-time yield of the targeted product is low, and a large multi-tube type of reactor is required.
One attempt at solving this problem has been to react an aldehyde and an alcohol in the presence of oxygen. For example, it has been reported that ethyl acetate is obtained by oxidizing ethanol in the presence of a metallic palladium catalyst (Kogyo Kagaku [Industrial Chemistry], Vol. 71, 1515 (1968)). According to this report, the reaction mechanism is such that the acetaldehyde produced by the oxidation of ethanol reacts with the ethanol as a result of the palladium catalyst, giving ethyl acetate.
A method has also been proposed for applying these oxidative esterification reactions to α,β-unsaturated aldehydes to manufacture α,β-unsaturated carboxylic esters. For instance, U.S. Pat. No. 4,249,019 discloses a method for manufacturing α,β-unsaturated carboxylic esters by reacting an aldehyde and an alcohol in the presence of a catalyst composed of the following three components: (1) palladium, (2) at least one member of the group consisting of lead, thallium, and mercury, and (3) at least one member of the group consisting of alkali metals and alkaline earth metals. U.S. Pat. No. 4,518,796 discloses a method for manufacturing α,β-unsaturated carboxylic esters by reacting the above-mentioned components in the presence of a catalyst composed of an intermetallic compound composed of palladium and bismuth.
Another known method for manufacturing carboxylic esters involves reacting an aldehyde with an alcohol in the presence of a catalyst comprising gold supported on a hydrophobic carrier such as activated carbon (Unexamined Japanese Patent Publication 2000-154164).
Meanwhile, methods in which a carboxylic ester is manufactured using an alcohol as the starting raw material in the presence of oxygen are also known. For instance, a method has been proposed in which an intermetallic compound including palladium and at least one member of the group consisting of lead, mercury, thallium, and bismuth is used as a catalyst (Japanese Patent Publication S62-7903).
However, the catalysts discussed in prior art all have low catalytic activity, and there is a need for further improvement in order to manufacture carboxylic esters more efficiently.